A box type PTC relay is an electrical part that guards electrical circuits from overcurrent or overtemperature situations by using a positive temperature coefficient (PTC) thermistor. Because it is housed in a plastic or metal box for safety and simpler installation. At typical operating temperatures, the PTC thermistor in the box type PTC relay has a low resistance when in use. The thermistor's temperature rises as a result of an increase in current flow, greatly increasing the resistance of the thermistor. The circuit is shielded against overcurrent situations by this increase in resistance, which restricts the current flow.